Carton



C. W. THIETS Dec. 11, 1962 CARTON 2 Sheets-Sheet 1 /C/c.

Filed Nov. l5, 1960 Dec. 1l, 1962 c. w. THIETs 3,067,923

yCARTON Filed Nov. 15, 1960 2 Sheets-Sheet 2 hired States This inventionrelates to disposable cartons which may be used either to contain orexclude liquid, and to shipping containers which include such cartons.The invention is particularly directed to self-sealing reinforcedparallelepipedal cartons which may be large enough to hold severalgallons of liquid Without leakage during extended shipment or storage.

Various more or less liquid impervious cartons have been known for manyyears. One example is the cornmon cardboard milk carton, in which aformed container is immersed in molten paraffin or similar material tocoat both inner and outer surfaces and thereafter filled with milk orsimilar dairy products. Milk cannot be stored for more than a few days,and such cartons are seldom shipped for very great distances.

Multi-ply cartons have previously been formed from cardboard laminatedto an inner liner of metallic foil, waxed paper, plastic film, etc.; allsuch containers heretofore known to me are subject to leakage ifconstructed larger than conventional milk cartons, used to containliquids, and subjected to extended shipment or storage. More recentlythere have been available multi-gallon parallelepipedal cartons in whicha flexible plastic bag is mounted in a corrugated cardboard shippingcontainer and sealed to one or more sides, at least at the portions ofthe side adjacent the corners. When such cartons are lled with liquidand shipped for very great distances, leaks invariably occur because ofthe repeated flexing of the plastic lm liner, especially at the top ofthe carton.

In accordance with my invention, I have devised a compact carton whichis completely hermetically sealed and which can be fabricated incapacities as great as gallons or more at costs far below those formetal or glass containers. When filled with liquid my novel cartonresists flexing for extended shipping periods. The usually vulnerablecorners where the sides intersect the top are protected, and thus even5-gallon or larger cartons embodying my invention are practically immuneto flex-induced leakage in the course of normal commerce.

My invention comprises a parallelepipedal carton which is completelyhermetically sealed, the carton being formed from relatively stiffmoisture-impervious sheet material. Each end of ythe carton is sealed bya folded closure formed from integral extensions of the sides of thecarton. I have overcome the problem of llex cracking lwhich hasheretofore plagued the industry by firmly atlixing between the end ofthe carton and the folded closure a separate flex-inhibiting stitfeningpad having slightly smaller dimensions than the inner cross-sectionaldimensions of the carton. The stiftening effect imparted by this padminimizes or prevents the vibrating or fluttering action which isotherwise characteristic of the top of such cartons when in transit, andthereby greatly inhibits the formation of leaks caused by flex crackingof the carton material at the corners formed by the intersection of thetop and two adjacent sides.

My invention Will be better understood by reference to the attacheddrawing, in which like numbers refer to like parts in the several viewsand in which:

FIGURE 1 is a plan view of a carton blank prior to set-up;

FIGURE 2 is a view in perspective of a partly formed carton, showing oneend open;

Patented Dec. 11, 1962 FIGURE 3 is a view in perspective of a partlyformed carton, showing the upper end closed and a stiflenhig pad readyto be applied;

FIGURE 4 is a cross-section of one type of seal for the carton; and

FIGURE 5 is a partially cut away view in perspective of the completelyformed carton surrounded by a corrugated sleeve and mounted in acorrugated box for shipping.

In FIGURE 1, the portion of the blank which will ultimately form theinside of the carton lies uppermost, the blank being divided verticallyinto 8 parts by fold lines I8, 19, 20, 2l, 22, 23, and 24. It issimilarly divided horizontally by base fold lines 25 and 25a, and Secondfold lines 26 and 26a. The portions of the blank lying above base line25 are folded to form the top of the carton, lwhile the portions of theblank lying below base line 25a are folded to form the bottom of thecarton. In the finished or semiinished carton the top closure is formedfrom half panels l2 and i3, which comprise one end, half panels I5 and16, which comprise the other end, and panels 14 and 1'7, which representopposite side panels. Half panels 12a, 13a, 15a and 16a., and sidepanels 14a and 17a are analogous panels used in forming the bottomclosure.

The blank shown in FIGURE l may be formed from a wide Variety ofmoistureor liquid-impervious materials; in certain instancesconventional wax-treated cardboard may be suitable, while in otherskraft paper which is extrusion-coated with polyethylene, nylon, or otherliquid-impermeable film-forming plastic or polymeric material, will bepreferred. Similarly polyvinyl chloride, polyvinyl alcohol and variousother films are useful under various appropriate conditions for specificapplications. I have found that an extremely suitable general purposeproduct may be formed from heavy kraft paper, 10 point white cupboard,potlatch paper, or the like, to which is laminated a membrane having anexposed polyethylene layer in conjunction with a tensilized polyesterlilm layer. The polyethylene provides heat sealing properties; thepolyester film prevents penetration of any liquid contents which mightseep through the polyethylene and imparts high mechanical strength anddimensional stability. A two layer membrane having a 11/2 mil (0.0015inch) film of polyethylene laminated to a 1/a-mill (0.0005 inch) film ofpolyethylene terephthalate film, such as Mylar polyester film, has beenfound highly effective, and will be referred to in describing theattached drawing. Other laminated membranes may be used to providespecific desired properties; e.g., aluminum vaporcoated 0.0005 inchpolyester film sandwiched between a 0.001 inch and a 0.0005 inchpolyethylene film may be used to retain the lavor or taste of thepackage contents or to provide an especially elfective barrier to oxygenor carbon dioxide.

The carton blank shown in FIGURE 1 is formed into an envelope, which canlater be converted into a parallelepipedal carton, by being folded alongline 21 in such a manner that iin panel 10 is placed in direct contactwith fin panel 1l, fin panels 31 and 31a being folded at their midpointsto be placed similarly in contact with themselves throughout theircoincident area. Fin panel 10 is then adhered or sealed to lin panel Il,and the two halves of tin panel 31a are sealed to each other, the upperend remaining open for convenience in filling. This envelope may beshipped in this condition to the ultimate consumer or, if desired, itmay be adhered inside a corrugated cardboard shipping container and theentire assembly folded flat.

In converting the envelope described in the preceding paragraph to acarton, the user first folds it at along lines 19, 20, 22, and 23 toprovide a rectangularly crosssectioned `body portion, fold lines 18 and24, and 21 tending to flatten out and conform to the straight sides onwhich they then appear. Because fin panel 31a is sealed to itselfthroughout its coincident area, the -bottom side panels 14a and 17a arethereby simultaneously lbent inwardly along fold line 25a, end panels12a- 13a and 15a- 16a being bent outwardly along fold line 25a. Theoutward folding of the end panels and the inward folding of the sidepanels causes half end panel 15a to be folded inwardly along diagonalline 29a and half panel 16a to be folded inwardly along diagonal line30a; by exactly the same process panel 12a is folded along line 27a andpanel 13a is folded along line 28a. The seal formed by adhering the twohalves of fin panel 31a to itself is then folded flat against the bottomof the carton, i.e., against side panel 14n. At this point, thepartially set up carton appears as in FIGURE 2, with the triangular earformed by the folding of end panel 12a- 13a shown extending, the secondear, formed from end panel 15a-16a not being shown but extending behindthe carton. The ears are then folded over the bottom and adheredthereto, preferably in the same manner as hereinafter described inconjunction with the top closure.

The carton as shown in FIGURE 2 is now filled with liquid or othercontents, and the folded top closure effected in much the same way asthe folded bottom closure. Side panels 14 and 17 are bent inwardly alongbase line 2S, while end panels 12-13 and 15-16 are bent out- Wardly,thus forming triangular ears by folding together the inner surfaces ofend half-panels 12, 13, 15, and 16 along diagonal lines 27, 28, 29, and30, respectively. The contacting inner surfaces of fin 31 are now sealedtogether typically by pressing them at 70 lbs. per square inch betweentwo 400 F. metal bars, after which fin 31 is folded into contact withside panel 14, as shown in FIGURE 3. In order to form a flat top and aneffective seal, the distance from base fold line 25 to fold line 26should be 1/2 the breadth of the package, i.e., 1/2 the distance betweenfold lines 19 and 2-3. Rigidifying and stitfening pad 40, preferablyconstructed of an appreciably thicker and stiffer material than thecarton, is now placed on the end of the carton, centered with respect tothe cross-section of the carton and adhered to fin 31 and side panels 14and 17. The triangular ears formed from end panels 12-13 and 15-16 arenow adhered to the upper surface of pad 40, thereby greatly rigidifyingthe upper end of the carton; if desired, pad 40 may be provided withslots to accommodate the ears and thus afiix pad 40 in position. thismanner is effectively freed from the problems of vibration, flutter, andflexing.

In FIGURE 4 a preferred Vertical side seal 10-11 is shown in greatlyenlarged cross-section. The blank of which side fin panels 10 and 11 arepart comprise kraft paper '32 coated with a laminated film of polyester33 and polyethylene 34. The fin seal itself is folded back against theside panel of the carton along fold line 18, as is indicated in FIGURES2, 3, and 5.

Although this reinforced carton has substantial utility, especially inhalf-gallon or smaller sizes, without further modification, the rigidityand leak proofness of both small and large cartons is still furtherenhanced by being mounted inside a corrugated cardboard box to form acomposite package. A particularly effective manner of accomplishing thisis shown in FIGURE in which the folded carton complete with stiffeningpad 49, is mounted inside corrugated sleeve 50 and corrugated box 60.The inner top tiaps 6l and 62 of box 60 are folded down over the top ofthe carton while outer box iiaps 63 and 64 are in turn folded over andsealed to fiaps 61 and 62.

In a specific preferred example, the polyester surface of a preformedcomposite film membrane consisting of 0.0005-inch tensilizedpolyethylene terephthalate polymer (Mylar) film bonded to 0.0015-inchpolyethylene film The top of a carton reinforced in was adhered to astrip of kraft paper having a thickness of 0.018 inch and a basis Weightof 58 lbs. per ream, and thereafter dried by passing it, film side in,around a 160 F. hot can. The laminating adhesive, conventionally usedfor adhering polyester films to paper, was the H, B. Fuller Companys8020, a 39% solids aqueous emulsion of a synthetic rubber-resin blend,applied in the amount of about 5 wet grains per Z4 square inches. Thestrip was cut into 39" x 26 rectangular blanks, creased as shown inFIGURE l. Each blank was then folded to form a square cross-sectionedcarton about 9 inches on each side and the bottom closure formed byfolding as hereinbefore described, some of the cartons having an 8% x8%" x 1A" 200 lb. burst strength A-fiute corrugated stiffening padinterposed between the bottom of the carton and the folded closure andbeing sealed thereto, and the other cartons being formed in the same waybut including no stiffening pad. Each carton was placed in a snug 3/16thick 275 lb. burst strength C-flute corrugated cardboard sleeve andplaced in a corrugated cardboard box (also 275 lb. C-flute) in themanner of FIGURE 5. Each carton was then filled with 5 gallons of watercontaining a fluorescent dye, after which the top was folded, closed,and sealed; stiffening padswere omitted or inserted, as appropriate, tomake the top and bottom closures substantially identical. The cardboardboxes were then sealed and the packages thereafter subjected to theprocedure for the vibration testing of packaged products weighing lessthan lbs. specified by the National Safe Transit Committee, 1145Nineteenth St. N.W., Washington 6, D.C. In this test, the package isplaced on the table of a Gaynes vibrator and the vibration frequencyadjusted so that the package leaves the table for at least 1/16 inch atsome interval during the vibration cycle. A satisfactory package mustnot develop leaks within one hour. Packages without stiffening pads alldeveloped leaks at one or more upper corners within 20 minutes, whilepackages having stiffening pads showed no trace of leakage after morethan 3 hours. Identical containers were filled with water, reinforcedwith stiftening pads, packed in corrugated cardboard boxes, and shippedfor over 700 miles by rail freight without the slightest indication ofleakage.

The carton of my invention has, as I have indicated, particular utilityin containing liquid, but it is also highly useful in the shipping ofmoisture-sensitive products, eg., ammonium nitrate, calcium chloride,isocyanate resin, etc. In such instances the resistance to cracking ofthis carton prevents moisture from penetrating and deleteriouslyaffecting the contents of the package.

My invention also contemplates various mechanical modifications of thecarton shown. For example, the triangularly folded end panel 15-16 shownin FIGURE 3 may be cut off `at some point intermediate fold line 25 andthe apex of the triangle to form a pouring spout, the remaining openportion being temporarily sealed by a crimped steel band or similardevice. Likewise, pouring spouts or similar fitments can be mounted inside panels 14 or 17 so that the contents of the package may bedispensed directly therefrom. In instances where the carton may befilled through the spout from which the contents will be dispensed, itmay be desirable to form a closed envelope, sealed not only along fins10-11 and 31a, but also along fin 31, thus eliminating the step offorming a top seal after filling the carton. Similarly, although thevertical fin seal facilitates carton fabrication and is thereforepreferably located as shown, it may be placed at a corner or at themiddle of one side. Lap seals may be suitable where the edge of thecarton exposed to liquid or moisture is provided `with a suitablebarrier, e.g., by extending the film liner around the edge.

What I claim is:

l. A completely hermetically sealed parallelepipedal containercomprising a rectangularly cross-sectioned carton yformed fromrelatively stiff moisture-impervious sheet material subject to failureby flex cracking, each end of said carton being sealed by a foldedclosure formed from integral extensionsof the sides of said carton, andincluding a separate flex-inhibiting stiffening pad of slightly smallerdimensions than the internal cross-section of said carton positionedparallel to said ends firmly aixed between said end 4and said foldedclosure and lying within the rectangle formed by imaginary extensions ofthe sides of said cartons.

2. A hermetically sealed rectangularly cross-sectioned carton formedfrom relatively stiff moisture-impervious sheet material subject todex-cracking, at least the top of said carton being structurally stienedand sealed by a closure including folded panels which are integralextensions of the body of said carton, and a rectangular Hex-inhibitingstilfening pad similar in dimensions to the internal horizontalcross-section of said body; said panels comprising two opposing endpanels and two opposing side panels connected at corner fold lines, eachpanel being connected to the corresponding major panel of the carton ata base fold line forming an edge of said closure, each panel having asecond fold line parallel to the base fold line and spaced therefrom adistance equal to one.

half the breadth of said closure, each end panel having third and fourthfold lines connecting the ends of its base fold line with the midpointof its second fold line; said side panels being folded toward each otherat 90 angles along their respective base lines, and the inner surface ofeach end panel being folded together along said third and fourth foldlines t0 form substantially triangular ears, so that corresponding innersurfaces of the portions of the panels lying beyond the second foldlines contact throughout the length lying between said midpoints andform a tin, contacting surfaces of said iin being hermetically sealedtogether, said fin being folded against one of said side panels; saidflex-inhibiting stilfening pad being of slightly smaller dimensions thanthe inner dimensions of said body and positioned over said fin and saidside panels in contact therewith and within the rectangle formed byimaginary extensions of said body, said ears being folded over andadhered to said pad.

3. A hermetically sealed package of liquid contents, highly resistant toleakage induced by vibration and comprising a rectangularlycross-sectioned carton formed from sheet material impervious to saidliquid, at least the top of said carton being structurally stiifened andsealed by a closure including folded panels which are integralextensions of the body of said carton, and a rectangular flex-inhibitingstii'fening pad similar in dimensions to the internal horizontalcross-section of said body; said panels comprising two opposing endpanels and two opposing side panels connected at corner fold lines, eachpanel being connected to the corresponding major panel of the carton ata base fold line forming an edge of said closure, each panel having asecond fold line parallel to the base fold line and spaced therefrom adistance equal to onehalf the breadth of said closure, each end panelhaving third and fourth fold lines connecting the ends of its base foldline with the midpoint of its second lfold line; said side panels beingfolded toward each other at 90 angles along their respective base lines,and the inner surface of each end panel being folded together along saidthird and fourth fold lines to form substantially triangular ears, sothat corresponding inner surfaces of the portions of the panels lyingbeyond the second fold lines contact throughout the length lying betweensaid midpoints and form a n, contacting surfaces of said iin beinghermetically sealed together, said iin being folded against one of saidside panels; said flex-inhibiting stiifening pad being of slightlysmaller dimensions than the inner dimensions of said body and positionedover said fin and said side panels in contact therewith and within therectangle formed by imaginary extensions of said body, said ears beingfolded over and adhered to said pad; said carton being snugly surroundedon at least four sides by a stiff t3 sleeve, the thus-surrounded cartonbeing in turn snugly positioned in a strong cardboard box.

4. A hermetically sealed parallelepipedal package of liquid contents,highly resistant to leakage induced by vibration and Which can beshipped for great distances without loss of the contents, and comprisinga rectan gularly cross-sectioned carton formed from sheet materialimpervious to said liquid, the top and bottom of said carton beingstructurally stiffened and sealed by a closure including folded panelswhich are integral extensions of the body of said carton, and arectangular flex-inhibiting stitfening pad similar in dimensions to theinternal horizontal cross-section of said body; said panels comprisingtwo opposing end panels and two opposing side panels connected at cornerfold lines, each panel being connected to the corresponding major panelof the carton at a base fold line forming an edge of said closure, eachpanel having a second fold line parallel to the base fold line andspaced therefrom a distance equal to one-half the breadth of saidclosure, cach end panel having third and fourth -fold lines connectingthe ends of its base fold line with the midpoint of its second foldline; said side panels being folded toward each other at angles alongtheir respective base lines, and the inner surface of each end panelbeing folded together along said third and fourth fold lines to formsubstantially triangular ears, so that corresponding inner surfaces ofthe portions of the panels lying beyond the second fold lines contactthroughout the length lying between said midpoints and form a iin,contacting surfaces of said fin being hermetically sealed together, saidiin being folded against one of said side panels; said dex-inhibitingstiffening pad being of slightly smaller dimensions than the innerdimensions of said body and positioned over said fin and said sidepanels in contact therewith and within the rectangle formed by imaginaryextensions of said body, said ears being folded over and aiiixed to saidpad.

5. A hermetically sealed parallelepipedal package of liquid contents,highly resistant to leakage induced by vibration and which can beshipped for great distances Without loss of the contents, and comprisinga rectangularly cross-sectioned carton, the top and bottom of saidcarton being structurally stiifened and sealed by a closure includingfolded panels which are integral extensions of the body of said carton,and a rectangular flex-inhibiting stiifening pad similar in dimensionsto the internal crosssection of said body; said panels comprising twoopposing end panels and two opposing side panels connected at corneryfold lines, each panel being connected to the corresponding major panelof the carton at a base fold line forming an edge of said closure, eachpanel having a Second fold line parallel to the base fold line andspaced therefrom a distance equal to one-half the breadth of saidclosure, each of said end panels having third and fourth fold linesconnecting the ends of its base fold line with the midpoint of itssecond fold line; said Side panels being folded toward each other at 90angles along their respective 'base lines, and the inner surface of eachend panel being folded together along said third and fourth fold linesto form substantially triangular ears, so that corresponding innersurfaces of the portions of the panels lying beyond the second foldlines contact throughout the length lying between said midpoints andform a iin, contacting surfaces of said fin being hermetically sealed together, said tin being folded against one of said side panels; saiddex-inhibiting stiifening pad being of slightly smaller dimensions thanthe inner horizontal dimensions of said body and positioned over saidtin and said side panels in contact therewith and within the rectangleformed by imaginary extensions of said body, said ears being folded overand adhered to said pad; said carton being snugly contacted on at leastfour sides by a corrugated cardboard sleeve which is in turn iirmlypositioned in a corrugated cardboard box; and aheat-andpressure-sealing, fluid-impervious, flexible, strong, selfdsupporting, preformed plastic film membrane covering the entire interiorsurface of said interconnected panels and effectively sealedthereagainst at least along all fold line areas and against the areasextending from said second fold lines to the free edge of said panels.

6. A hermctically sealed parallelepipedal package of liquid contents,highly resistant to leakage induced by vibration and which can beshipped for great distances Without loss of the contents, and comprisinga rectangularly cross-sectioned carton, the top and bottom of saidcarton being structurally stiifened and sealed by a closure includingfolded panels which are integral extensions of the body of said carton,and a rectangular Hex-inhibiting stiffening pad similar in dimensions tothe internal cross-section of said body; said panels comprising twoopposing end panels and two opposing side panels connected at cornerfold lines, each panel being connected to the corresponding major panelof the carton at a base fold line forming an edge of said closure, eachpanel having a second fold line parallel to the base fold line andspaced therefrom a distance equal to one-half the breadth of saidclosure, each of said end panels having third and fourth fold linesconnecting the ends of its base fold line with the midpoint of itssecond fold line; said side panels being folded toward each other at 90angles along their respective base lines, and the inner surface of eachend panel being folded together along said third and fourth fold linesto form substantially triangular ears, so that corresponding innersurfaces of the portions of the panels lying beyond the second foldlines Contact throughout the length lying between said midpoints andform a iin, contacting surfaces of said iin being hermctically sealedtogether, said iin being folded against one of said side panels; saidflexinhibiting stifening -pad being of slightly smaller dimensions thanthe inner horizontal dimensions of said body and positioned over saidiin and said side panels in contact therewith and within the rectangleformed by imaginary extensions of said body, said ears being folded overand axed to said pad; said carton being snugly contacted on at leastfour sides by a corrugated cardboard sleeve which is in turn rmlypositioned in a corrugated cardboard box; and aheat-and-pressure-sealing, fluidimpervious, flexible, strong, thin,plastic layer covering the entire interior surface of saidinterconnected panels and effectively sealed thereagainst at least alongall fold line areas and against the areas extending from said secondfold lines to the free edge of said panels.

7. A hermctically sealed rectangularly cross-sectioned carton formedfrom relatively stiff moisture-impervious sheet material subject tohex-cracking, at least the top of said carton being structurallystiffened and sealed by a closure including folded panels which areintegral extensions of the body of said carton, and a rectangularHexinhibiting stiffening pad similar in dimensions to the internalcross-section of said body; said panels comprising two opposing endpanels and two opposing side panels connected at corner fold lines, eachpanel being connected to the corresponding major panel of the carton ata base fold line forming and edge of said closure, each panel having asecond fold line parallel to the base fold line and spaced therefrom adistance equal to one-half the breadth of said closure, each of said endpanels having third and fourth fold lines connecting the ends of itsbase fold line with the midpoint of its second fold line; said sidepanels being folded toward each other at 90 angles along theirrespective base lines, and the inner surface of each end panel beingfolded together along said third and fourth fold lines to formsubstantially triangular ears, so

that corresponding inner surfaces of the portions of the panels lyingbeyond the second fold lines contact throughout the length lying betweensaid midpoints and form a fin, contacting surfaces of said n beinghermctically sealed together, said iin being folded against one of saidside panels; said flex-inhibiting stiffening pad being of slightlysmaller dimensions than the inner horizontal dimensions of said body andpositioned over said iin and said side panels in contact therewith andwithin the rectangle forrned by imaginary extensions of said body, saidears being folded over and aflixed to said pad; said carton being snuglycontacted on at least four sides by a strong, stiff sleeve which is inturn firmly positioned in a strong, stiff box; and aheat-and-prcssure-sealing, fluidimpervious, flexible, strong, thin,plastic layer covering the entire interior surface of saidinterconnected panels and effectively sealed thereagainst at least alongall `fold line areas and against the areas extending from said secondfold lines to the free edge of said panels.

8, A hermctically sealed parallelepipedal package of liquid contents,highly resistant to leakage induced by vibration and which can beshipped for great distances without loss of the contents, and comprisinga rectangularly cross-sectioned carton, the top and bottom of saidcarton being structurally stiffened and sealed by a closure includingfolded panels which are integral extensions of the body of said carton,and a rectangular flex-inhibiting stiffening pad similar in `dimensionsto the internal crosssection of said body; said panels comprising twoopposing end panels and two opposing side panels connected at cornerfold lines, each panel being connected to the corresponding major panelof the carton at a base fold line forming an edge of said closure, eachpanel having a second fold line parallel to the base fold line andspaced therefrom a distance equal to one-half the breadth of saidclosure, each of said end panels having third and fourth fold linesconnecting the ends of its base fold line with the midpoint of itssecond fold line; said side panels being folded toward each other atangles along their respective base lines, and the inner surface of eachend panel being folded together along said third and fourth fold linesto form substantially triangular ears, so that corresponding innersurfaces of the portions of the panels lying beyond the second foldlines contact throughout the length lying between said midpoints andform a 1in, contacting surfaces of said fin being hermctically sealedtogether, said n being folded against one of said side panels; saidflex-inhibiting stifening pad being of slightly smaller dimensions thanthe inner horizontal dimensions of said body and positioned over saidfin and said side panels in contact therewith and within the rectangleformed by imaginary extensions of said body, said ears being folded overand aft'ixed to said pad; and a heat-and-pressuresealing,fluid-impervious, flexible, strong, self-supporting, preformed plasticlm membrane covering the entire interior surface of said interconnectedpanels and effectively sealed thereagainst at least along all fold lineareas and against the areas extending from said second `fold lines tothe free edge of said panels.

References Cited in the file of this patent UNITED STATES PATENTS2,326,649 Howard Aug. 10, 1943 2,331,010 Waters Oct. 5, 1943 2,337,730Berch Dec. 28, 1943 2,372,452 Rohdin Mar. 27, 1945 2,944,715 VergobbiJuly 12, 1960

